Police said that the victim in his late 50s was walking his dogs up past Skerton Bridge and onto the grassed area behind Bridge House, Lancaster.
As he walked towards a bin to dispose of his dog’s waste the male suspect approached him, shouting that the victim needed to pick up his dog waste.
He then brought out an orange-coloured mobile phone and asked the victim to smile whilst he took an image of him before punching him in the face.
The suspect then walked off and urinated in the nearby bushes.
The victim was left with a deep cut to his forehead that required stitches at the Royal Lancaster Infirmary.
The suspect is described as a white male, in his mid 50s, approximately 5’8” in height, medium build with a Northern accent, shaven face wearing a black bobble hat, black leather jacket and dark or black trousers holding a bright orange coloured mobile phone.
PC Simon Harrison, from Lancaster & Morecambe Police, said: “We would appeal to the public and the to the individual directly, to get in touch with us if you believe that you can help us and provide any further information in regards to this investigation”.
The incident happened at 6pm on March 11.
Anybody with any information can call police on 101 quoting Crime Number WB1501878 or Crimestoppers anonymously on 0800 555 111 or on-line at Crimestoppers-uk.org.
No personal details are taken.